Case in point: Robby Cooksey, 9, who stuffed his LL Cool J tape and his favorite World Wrestling Federation card into his size 4 sneakers, dropped them to the sand and, despite a raging case of poison ivy he picked up at camp, plunged into the 55-degree water of Lake Michigan. But why?

But to most, the bite of the water was like a guard dog at the shoreline, making it light duty for the skeleton staff of lifeguards working their first day Saturday at Chicago Park District beaches.

Although the beaches do not officially open with a full complement of lifeguards until June 18, the district estimated that the two most popular beaches, at Oak Street and North Avenue, had drawn crowds exceeding 55,000 by 5 p.m. Saturday.

And there were other signs at the lakefront that summer was indeed making its way through its traditional Memorial Day gateway. A smiling girl pinched a butterfly by its wings and pulled it from the grip of the water`s surface tension; a couple skewered kebabs as their coals burned low; and children rediscovered the science of screwing their feet deep into the sloppy sand at the water`s edge.

Robert Pless, 24, a high school teacher from St. Louis, sat at the water`s edge grading a pile of 10th-graders` final examinations on literature, from Chaucer through the Modernists.

``These first two are by students who are not very gifted,`` he said.

``But they are definitely benefitting from the scenery.``

As distant sailboats heeled away from a steady 9- to 16-knot southwesterly wind, discussions at the beach turned to the precise temperature of the water.

``You have to chip the ice off before you dive in,`` snapped a very dry passerby.

A more objective measurement, by the Central Weather Service, put the official temperature of the lake`s water Saturday at 55 degrees at the 68th Street crib.

The official high temperature for Saturday was 88 degrees at O`Hare, but an unofficial 89 was recorded at the lakefront. Sunday and Monday were expected to be just about the same, according to the service.

All three weekend days promised to be perfect for, say, burying your best friend in the sand. Corey Tripp, 10, was quite literally up to his neck in it Saturday as his buddy piled it on.

``When I get out, then I get to bury him,`` Corey said, gesturing with his head at his friend.

And at the beach, this is exactly what the Memorial Day weekend is all about.

Further inland, at Oakwood Cemetery, Nancy Phillips, 72, leaned on a spade Saturday as she pressed the slender staff of an American flag among the cluster of stems that marks the grave of her son, a veteran. She held another flag in her hand for the nearby grave of her husband, also a veteran.

``It`s a veterans` holiday,`` she said with a light but firm smile.

Her great-grandson, Mario, 9, sweated lightly beside her as he clutched a cool ginger ale in Saturday`s heat. ``I`m going to the beach Monday,`` he said.